A dual chamber pacing system offers the significant advantage of providing AV-synchrony when the atrial rate can be tracked, i.e., is physiologic. Thus, every time an atrial beat is sensed at a physiological rate, the pacemaker delivers a synchronized ventricular pace pulse in the absence of a timely natural ventricular beat. Such a pacemaker system generally has some form of upper tracking limit, such as a dynamic tracking limit disclosed in U.S. Pat. Nos. 5,531,771 and 5,549,648, assigned to the same assignee as this invention, and incorporated herein by reference. Whenever the P-wave is sensed at a rate below the tracking limit, the synchronous ventricular pace pulse is prepared by timing out an appropriate AV interval following the occurrence of the P-wave. For a sensed P-wave at a higher rate, pacemaker may provide for Wenckebach operation, but generally higher rate non-physiological atrial signals are not tracked. AV synchrony can also be lost due to other circumstances, such as the sensing of a PVC or a pacemaker determination of ongoing retrograde conduction. Since AV synchrony is highly desirable, pacemaker systems attempt to re-establish such synchronous operation as soon as possible after a loss of tracking. One technique for doing this, which is disclosed in the above referenced patent, is the delivery of an atrial synchronization pulse (ASP), which is used in DDD systems having atrial and ventricular leads.
The problem of re-establishing AV synchrony after loss of tracking is more acute in a VDD pacing system having a single pass lead. As is known, the single pass lead has one or two electrodes to pace and sense in the ventricle, and a pair of floating ring electrodes positioned on the lead so that they are placed in the atrium where they sense P-waves.
While a VDD pacing system is designed to pace only in the ventricle, it has been observed in patients that pacing through the atrial rings in a single pass lead VDD system may also provoke atrial contraction. However, because the rings are floating in the atrium, the stimuli may not be effective. In the extreme, where these rings are positioned far from the atrial wall, no atrial pacing may be possible; where the rings are fortuitously placed closer to the atrial wall, most of the delivered atrial pace stimulus pulses may be effective. Whether or not atrial pacing is reliable in such a single pass lead system either for occasional delivery of ASPs or more generally for ongoing constant atrial pacing, is a circumstance that will vary from patient to patient. Likewise, a dual pacing system with an atrial lead as well as a ventricular lead may not have 100% atrial pacing efficiency, due to atrial lead dislodgement, or too high chronic atrial pacing threshold.
Accordingly, an objective of this invention is to provide a pacemaker having an algorithm particularly applicable to single pass VDD lead systems for determining when an atrial pace should be delivered; providing pacing through the atrial electrodes; and evaluating whether or not the pace was effective. Through such ongoing evaluation over a number of atrial paces, the pacemaker system can adjust the atrial pace pulse output level either lower or higher, or may even disable atrial pacing if the efficiency is too low. If pacing efficiency is high enough, the pacemaker can operate in the DDD mode. The invention further aims to provide diagnostic counters for an indication of the atrial pacing effectiveness, so that the physician, through an external programmer, can determine a desired pacing mode.